418 The American Naturalist. [May, 



phenocrysts of sanidine and also sodalite and eleolite in a fine-grained 

 groundmass of eleolite, orthoclase and minute idiomorphic pyroxenes 

 and amphiboles that sometimes shows a fluidal structure. Because of 

 their porphyritic structure and their close relationship with the eleo- 

 lite-syenite they are classed by the writer among the tinguaites. 



The augitic rocks associated with the eleolite-syenites in this region 

 are classed in Rosenbusch's new division of dyke rocks monchiquite. 

 They are all dark in color, and all are characterized by the possession 

 of phenocrysts of augite or of biotite. Those containing olivine are 

 placed with monchiquite, while of the non-olivinitic varieties two new 

 groups are formed; one, called fourchite, possesses phenocrysts of 

 augite ; in the other, ouachitite, biotite occurs in large quantity. In 

 the former augite constitutes nearly 75% of the entire rock. Its ground 

 mass is now crystalline, but its structure is thought to be the result of 

 the devitrification of a glassy base. In the amphibole ouachititee the 

 phenocrysts are biotite, hornblende and a fewaugites in a fine-grained 

 but originally a glassy groundmass. Augite is still the most promi- 

 nent bisilicate, but it now constitutes scarcely 20% of the rock mass. 

 The groundmass is composed of minute hornblende and augite crys- 

 tal-, with much magnetite and small, highly refractive grains of what 

 the author supposed to be sphene. The base contains many small, 

 lath-shaped crystals of feldspar. In addition to these two members of 

 the monchiquite group, there is probably a true monchiquite at the 

 south end of Allis Mt., in the same district. This rock is remarkable 

 for its great number of sphene grains and for the common occur- 

 rence in it of pseudomorphs of biotite after augite. 



At a few places where the contact of the syenite with the surround- 

 ing shales may be studied, the latter are found to be much changed 

 and to have developed in them small, irregularly bounded feldspars, a 

 iotite, gra 



The sequence of the e 

 syenite and pulaskite (same magma), fourchite, pegmatite and miaro- 

 litic dykes. 



In the Saline County region the rocks are not very different froaa 

 those of Fourche Mt. The prevailing gray syenite is coarser in gram 

 than that of Fourche Mt., and its orthoclase crystals (intergrown with 

 albite) tend to become porphyritic. The eleolite is also sometimes jn 

 large crystals. A plagioclastic variety of the i 



5 granitic than the orthoclase variety. The pyroxene 

 ad of diopside. Pulaskite is absent from this region, 

 3 occurs a porphyritic syenite with large orthoclase cryi 



is aeginte 

 but in its 



stals, crys- 



